The present invention relates to a thin imaging apparatus having large area for reading the distribution of signal charge quantity generated and stored in a photoconductive layer by incidence of photons and for generating an electric signal corresponding to the spatial distribution of the quantity of incident light, and relates to an operation method thereof.
A photoconductive image pickup tube is well known as an imaging apparatus which has a photoconductive layer for generating and storing signal charges according to the quantity of incident light and which reads out the signal charges generated and stored in the photoconductive layer into an external circuit in a time series form by using an electron beam and-generates an electric signal corresponding to the spatial distribution of the quantity of incident light. FIG. 5 is a schematic diagram showing the basic structure and operation principle of the photoconductive image pickup tube. An electron beam 502 emitted from a cathode electrode 501 is accelerated by a mesh electrode 503 to scan a photoconductive layer 504 under the control of electrostatic and/or electromagnetic deflection and focusing means (not illustrated). The electron beam scanning side, i.e., scanned surface, of the photoconductive layer 504 has a material and/or structure hard of emitting secondary electrons. When the scanning electron beam 502 arrives at the scanned surface, the potential of the scanned surface gradually falls. If the potential of the scanned surface becomes lower than that of the cathode electrode 501, however, the scanning electron beam cannot further arrive at the scanned surface. Immediately after it has been subjected to electron beam scanning, therefore, the potential of the scanned surface balances that of the cathode electrode 501. Target voltage V.sub.T, which is positive with respect to the cathode potential, is applied to a transparent electrode 505. Therefore, an electric field so oriented as to be positive on the substrate side and negative on the scanned surface side is applied to the photoconductive layer 504. If incident light 506 is applied from the outside to the photoconductive layer 504 under this state, as many electron-hole pairs as determined by the quantity of incident light are generated in the photoconductive layer. The above described electric field makes electrons run to the substrate side and makes holes run to the scanned surface side. The potential of the scanned surface is gradually raised from the cathode potential by holes which have arrived at the scanned surface. When the scanning electron beam 502 arrives at the scanned surface subsequently, the potential of the scanned surface is reset to the cathode potential again. At that time, stored signal charge depending upon the quantity of incident light at a pertinent location flows through a load resistor 507. By means of electron beam scanning, therefore, time-series electric signal corresponding to the spatial distribution of the quantity of incident light is obtained from an output terminal 508. In FIG. 5, numeral 509 denotes a transparent substrate, and numeral 510 denotes an electron gun tube for vacuum seal. Operation principle of a photoconductive imaging tube is disclosed in JP-A-58-194231, for example.
As described above, a photoconductive imaging tube has a single electron emitter. In JP-A-55-25910, however, a plurality of electron emitters having negative electron affinities which can be controlled respectively independently are disclosed. By using this, a second conventional technique in which a target of a vidicon is scanned in a time division manner by a plurality of electron beams projected one after another, for example, has been disclosed.
However, the above described photoconductive imaging tube needs magnetic and/or electric deflecting and focusing means, such as a coil for deflecting and focusing an electron beam emitted from the single electron emitter and thereby scanning the photoconductive target, and a cylindrically patterned electrode. This results in a problem that the distance between the photoconductive target and the electron emitter is long and hence a thin imaging apparatus cannot be obtained.
Furthermore, in an apparatus using the above described second conventional technique, the quantity of emitted electrons is controlled by changing the potential of the electron emitter itself and it is impossible to make electrons arrive at the above described photoconductive target by emitting and/or accelerating electrons. As described in detail by referring to FIG. 5, the potential of the scanned surface in a photoconductive imaging tube immediately after electron beam scanning balances the potential of the cathode electrode. During a storage interval lasting until that place is subjected to electron beam scanning again, the potential is gradually raised by the signal charge generated by incident light. Typically, the value of this potential rise is approximately several volts. If an imaging tube, for example, has a size of 2/3 inch, a signal current of 200 nA, storage time of 1/60 sec, and a photoconductive layer made of amorphous Se having a thickness of 4 .mu.m, then the potential of the scanned surface rises approximately 4 Volt during the storage interval. Since the potential rise of the scanned surface of the photoconductive target is thus small, it is extremely difficult to sufficiently extract electrons emitted from the cathode and make them arrive at the scanned surface. As a result of study made by the present inventors, such a configuration that a plurality of electron emitters are only disposed opposite to the photoconductive target as described above has been found to have the following problems. That is to say, it is difficult to make a sufficient amount of electron beams incident upon the scanned surface and control the quantity of incidence. Furthermore, since the electron beam emitted from the electron emitter is not sufficiently accelerated, the configuration is poor in property of going straight and beam bending is apt to cause resolution degradation and image distortion.
Furthermore, the present inventors have found that the apparatus using the above described second conventional technique has a problem that noise is caused by dispersion among the quantities of electrons emitted from electron emitters.
Furthermore, it has been found that the conventional photoconductive imaging tube and the apparatus using the above described second conventional technique has the following problems. That is to say, if it is attempted to obtain a thin imaging apparatus having a shortened distance between the photoconductive target and the electron emitter or an imaging apparatus, then the electrostatic capacity between the transparent electrode and the electron emitter and/or mesh electrode becomes large and hence degraded response increases the lag, resulting in one problem. If the quantity of incident light is large, then saturation of the output signal current due to insufficient quantity of electron beam causes a narrow dynamic range, resulting in another problem.